There is no guaranteed way to make your readers cry when you want them to. It's not like there's a box of story-elements you can just reach into that says "Crying" on it. And people get emotional over different things, so it can be hard to predict what they'll react to anyway.
However, there are a few things you can do to make your emotional scenes perhaps impact better. And this goes for emotional scenes in general, not just tragedy.
Make the readers invested in the characters
Spend time in the story on making your readers familiar with a character, and give them reasons to care. Maybe your character is on some kind of important mission, and if they fail something bad will happen; your readers will care about the success of the mission. Craft characters that have something they care about - that way, readers will have a higher chance of caring about them.
Raise the stakes
So your character has an important mission, and failure means bad things happening. How can you turn that into an emotionally impactful scene?
Threaten failure.
Give your character a challenge to their mission. Maybe someone steps in and tries to stop them, maybe they lose something important that could have helped them, maybe they tried to complete their mission and failed for some reason, and now they have to race against time to get to try again.
Deliver on your promises
If you've been threatening failure, have it happen. If you've been writing the story in such a way that one character has been in mortal danger for a long time - maybe someone is out to kill them? - then deliver on that. Have the trap snap shut.
A little goes a long way
There's a scene in the absolute travesty that is the live-action DragonBall movie, in which Goku falls to his knees beside the corpse of his grandfather - whom we've, uh, hardly seen on screen ^_^; - and raises his hands to the sky, screaming "NOOOOOOOOOOOOOOOO!" at the top of his lungs. It's terrible, it's badly written, and it's horrendously over-acted - and as a result, I just don't care.
Sometimes, you get a bigger emotional impact if you do something small. Sometimes a bundle of flowers on a gravestone is more emotional than someone clinging to a dead body bawling their eyes out.